1. Filed of the Invention
The present invention relates to an improvement in an injecting apparatus equipped with a check valve for preventing the reverse flow of a melt of molding materials.
2. Description of the Related Art
A typical prior screw injecting apparatus is shown in cross section in FIG. 7 hereof. The screw injecting apparatus 100 generally comprises a heating cylinder 101, a screw 102 received in the heating cylinder 101 rotatably and movably back and forth, a check valve 103 mounted on the tip end of the screw 102, an injecting cylinder 104 for moving the screw 102 back and forth, and a rotating means or device 106 for rotating the screw 102 via a piston rod 105 of the injecting cylinder 104.
In the injecting apparatus 100 thus arranged, plasticizing-metering phase, waiting phase and injecting phase are carried out in a manner as described below.
Plasticizing-metering phase: Raw molding materials are fed from a hopper 107 into the heating cylinder 101 during rotation of the screw 102 and heated by the heating cylinder 101 while being transferred toward a discharge nozzle 108 by rotation of the screw 102. By friction heat arising from the transfer and heat transmitted from the heating cylinder 101, the molding materials are plasticized and kneaded.
During that time, the check valve 103 is in the open state and thus permits passage therethrough of a melt of molding materials.
The screw 102 is pushed back rightward in FIG. 7 by a counter force of the melt accumulated around the tip end of the screw 102 (between the check valve 103 and the nozzle 108). The amount of melt can be metered by measuring the retreating or backward stroke of the screw 102.
Waiting phase: After completion of the metering, the screw 102 is held inactive until the melt becomes ripe for injection.
Injecting phase: By activating the injecting cylinder 104, the screw 102 is rapidly advanced at one stroke to cause a metered amount of melt accumulated forward of the screw 102 to be injected through the nozzle 108 into a mold not shown. Advancing movement of the screw 102 causes the check valve 103 to close, so that the melt is prevented from flowing back toward the screw 102.
FIGS. 8A and 8B are views illustrative of the structure and operation of the check valve 103 of the conventional screw injecting apparatus 100.
As shown in FIG. 8A, the check valve 103 is provided on the tip end of the screw 102 and includes a screw head 111 attached to the tip end of the screw 102 and a cylindrical check ring 112 slidably mounted on the screw head 111.
In the metering phase, the check valve 103 is in the open state so that rotation of the screw 102 causes a melt of molding materials to pass through a passage 113 in the check valve 103 in the direction of the arrows {circle around (1)} and {circle around (2)} shown in FIG. 8A.
In the injecting phase, as shown in FIG. 8B, the screw 102 is rapidly advanced as indicated by the profiled arrow {circle around (3)}. As the screw 102 advances, the check ring 112 is subjected to a force or pressure applied from a metered amount of melt 11 accumulated forward of the screw head 111. This causes a backward movement of the check ring 112 as indicated by the arrow {circle around (4)}, which continues until the check ring 112 comes into abutment with a valve seat 115 formed on a shoulder of the screw 102 (S1 shown in FIG. 8A is zero). The check valve 103 is thus closed so that the reverse flow of the melt 114 toward the screw 102 does not take place.
However, from the start of backward movement of the check ring 112 to the arrival of the check ring 112 at the valve closing position shown in FIG. 8B, the passage 113 is still in an open state. Accordingly, the melt 114 can flow backward through the passage 113 to the screw 102 side. Even though the amount of melt 114 flowing backward is very small, such backflow of the melt 114 deteriorates the metering accuracy.
In order to increase the injecting speed and metering accuracy, the amount of melt 114 flowing backward should be reduced to near zero.
It is accordingly an object of the present invention to provide an injecting apparatus equipped with a check valve which is capable of reducing the amount of backflow of the melt to nearly zero.
To achieve the foregoing object, according to the present invention, there is provided a screw infecting apparatus comprising: a heating cylinder for heating a molding material being transferred therethrough, the heating cylinder having a nozzle at one end thereof; a screw received in the heating cylinder rotatably and movably back and forth and having a tip end portion spaced backward from the nozzle, the screw, when rotating in one direction, feeding a melt of molding material toward the nozzle while the screw is pushed back by a reaction of the melt accumulated around the tip end portion of the screw head; and a check valve for preventing the melt from flowing backward toward the screw. The check valve comprises a seat ring mounted on the tip end portion of the screw and movable in unison with the screw, the seat ring having an axial groove for the passage therethrough of the melt, and a check ring mounted on the tip end portion of the screw in tandem relation to the seat ring and rotatable relatively to the screw and the seat ring within a predetermined angular range, the check ring having an axial groove for the passage thererthrough of the melt, the axial groove of the check ring and the axial groove of the seat ring being phased such that rotation of the screw in said one direction causes the axial groove in the check to communicate with the axial groove in the seat ring, thereby permitting passage of the melt through the check valve, and rotation of the screw in the reverse direction causes the axial groove of the seat ring to be closed by a body of the check ring, thereby blocking passage of the melt through the check valve. The screw injecting apparatus further includes a control unit for controlling operation of the screw such that in a metering phase of molding process, the screw rotates in said one direction to feed a melt of molding material toward the nozzle, when a metered amount of melt is accumulated forward of the tip end portion of the screw, the screw moves backward by a predetermined distance to reduce the reaction from the metered melt and, thereafter, the screw rotates in the reverse direction to cause the screw to move forward until the amount of forward movement of the screw becomes substantially equal to said predetermined distance.
When the screw is rotating in one direction (normal direction), the check valve is open, so that a melt of molding material is fed toward the nozzle and accumulated forward of the tip end portion of the screw disposed backward of the nozzle. The screw while rotating in the normal direction is pushed back by a reaction of the meld accumulated around the tip end portion of the screw. By measuring the stroke of the screw in the backward direction, it is possible to meter the amount of melt accumulated forward of the tip end portion of the screw.
When the metered amount of melt is accumulated forward of the tip end portion of the screw, the check valve may be closed in preparation for the injecting process by rotating the screw in the reverse direction. In this instance, however, reverse rotation of the screw causes the screw to move forward as if the screw bits into the metered melt accumulated around the tip end portion of the screw. With this forward movement of the screw, the metered melt is forced to leak out from the nozzle.
To avoid this problem, according to the present invention, when the metered amount of melt is accumulated forward of the tip end portion of the screw, the screw is moved backward by a predetermined distance under the control of the control unit. With this backward movement of the screw, the pressure in a melt accumulating chamber defined between the nozzle and the check valve is reduced. Then, the screw is rotated in the reverse direction to move the screw itself to move in the forward direction until the amount of forward movement of the screw becomes substantially equal to the predetermined distance of backward movement of the screw achieved previously. With this forward movement, the screw can assume the position achieved at the end of the metering phase or operation, so that the leakage of the melt from the nozzle does not take place. Additionally, since the check valve is closed in preparation for the injecting process, the amount of reverse flow of the melt does not take place during the injecting process. This enables a high-speed injecting process with improved metering accuracy.
In one preferred form of the present invention, the tip end portion of the screw has a plurality of evenly spaced driving teeth formed on an outer circumferential surface thereof, the check ring has a plurality of evenly spaced driven teeth formed on an end face thereof for meshing engagement with the driving teeth of the tip end portion of the screw. The axial groove of the check is formed in an outer circumferential surface of the check ring, and the axial groove in the seat ring is formed in an inner circumferential surface of the seat ring.
It is preferable that the control unit determines the amount of forward movement of the screw caused by reverse rotation of the screw on the basis of a correlation established between the amount of reverse rotation of the screw and the amount of forward movement of the screw.
The above and other objects, features and advantages of the present invention will become manifest to those versed in the art upon making reference to the following description and accompanying sheets of drawings in which a certain preferred structural embodiment incorporating the principle of the invention is shown by way of illustrative example.